Tuberculosis (TB) is a chronic infectious disease caused by infection with Mycobacterium tuberculosis and other Mycobacterium species. It is a major disease in developing countries, as well as an increasing problem in developed areas of the world.
More than 2 billion people are believed to be infected with TB bacilli, with about 9.2 million new cases of TB and 1.7 million deaths each year. 10% of those infected with TB bacilli will develop active TB, each person with active TB infecting an average of 10 to 15 others per year. While annual incidence rates have peaked globally, the number of deaths and cases is still rising due to population growth (World Health Organisation Tuberculosis Facts 2008).
Mycobacterium tuberculosis infects individuals through the respiratory route. Alveolar macrophages engulf the bacterium, but it is able to survive and proliferate by inhibiting phagosome fusion with acidic lysosomes. A complex immune response involving CD4+ and CD8+ T cells ensues, ultimately resulting in the formation of a granuloma. Central to the success of Mycobacterium tuberculosis as a pathogen is the fact that the isolated, but not eradicated, bacterium may persist for long periods, leaving an individual vulnerable to the later development of active TB.
Fewer than 5% of infected individuals develop active TB in the first years after infection. The granuloma can persist for decades and is believed to contain live Mycobacterium tuberculosis in a state of dormancy, deprived of oxygen and nutrients. However, recently it has been suggested that the majority of the bacteria in the dormancy state are located in non-macrophage cell types spread throughout the body (Locht et al, Expert Opin. Biol. Ther. 2007 7(11):1665-1677). The development of active TB occurs when the balance between the host's natural immunity and the pathogen changes, for example as a result of an immunosuppressive event (Anderson P Trends in Microbiology 2007 15(1):7-13; Ehlers S Infection 2009 37(2):87-95).
A dynamic hypothesis describing the balance between latent TB and active TB has also been proposed (Cardana P-J Inflammation & Allergy—Drug Targets 2006 6:27-39; Cardana P-J Infection 2009 37(2):80-86).
Although an infection may be asymptomatic for a considerable period of time, the active disease is most commonly manifested as an acute inflammation of the lungs, resulting in tiredness, weight loss, fever and a persistent cough. If untreated, serious complications and death typically result.
Tuberculosis can generally be controlled using extended antibiotic therapy, although such treatment is not sufficient to prevent the spread of the disease. Infected individuals may be asymptomatic, but contagious, for some time. In addition, although compliance with the treatment regimen is critical, patient behaviour is difficult to monitor. Some patients do not complete the course of treatment, which can lead to ineffective treatment and the development of drug resistance.
Multidrug-resistant TB (MDR-TB) is a form which fails to respond to first line medications. 5% of all TB cases are MDR-TB, with an estimated 490,000 new MDR-TB cases occurring each year. Extensively drug-resistant TB (XDR-TB) occurs when resistance to second line medications develops on top of MDR-TB. It is estimated that 40,000 new cases of the virtually untreatable XDR-TB arise annually (World Health Organisation Tuberculosis Facts 2008).
Even if a full course of antibiotic treatment is completed, infection with M. tuberculosis may not be eradicated from the infected individual and may remain as a latent infection that can be reactivated.
In order to control the spread of tuberculosis, effective vaccination and accurate early diagnosis of the disease are of utmost importance.
Diagnosis of latent TB infection is commonly achieved using the tuberculin skin test, which involves intradermal exposure to tuberculin protein-purified derivative (PPD). Antigen-specific T cell responses result in measurable induration at the injection site by 48-72 hours after injection, which indicates exposure to mycobacterial antigens. Sensitivity and specificity have, however, been a problem with this test, and individuals vaccinated with BCG cannot always be easily distinguished from infected individuals (this is particularly important in light of the fact that BCG does not protect against latent infection). In general, individuals who have received BCG but are not infected by M. tuberculosis show a PPD reaction below 10 mm in diameter whereas people who have a PPD reaction above 10 mm in diameter are considered to have been infected by M. tuberculosis. However, this rule is not applicable to individuals with immunosuppression due to HIV infection, which may result in a PPD reaction below 10 mm in diameter); or in endemic countries, where people infected by non-tuberculosis mycobacteria can show a PPD reaction above 10 mm in diameter.
Progress over recent years has seen the development of in vitro T cell based assays, based on interferon-gamma release and using antigens which are more specific to M. tuberculosis than PPD, namely ESAT-6 and CFP-10. These high specificity tests appear to be at least as sensitive as the tuberculin skin test and also demonstrate less cross-reactivity due to BCG vaccination. See Pai M et al Expert Rev. Mol. Diagn. 2006 6(3):413-422 for a recent review of latent TB diagnosis. However, since ESAT-6/CFP-10 are early stage antigens, assays based on ESAT-6/CFP-10 may only perform optimally in recently infected people. Consequently, the identification of new antigens specifically associated with latent tuberculosis may aid the development of more sensitive assays that could detect longer-term latent infections.
There remains a need for effective strategies for the treatment and prevention of tuberculosis, in particular the treatment and prevention of latent TB and the prevention of reactivation of TB.